Electromagnetic waveguide systems



% Aug. 26,1958 J. L. AYERS ETAL 2,849,688

ELECTROMAGNETIC WAVEGUIDE SYSTEMS L Filed Oct. 27, 1953 3 Sheets-Sheet l FIG. 1. (b)

FIG. 3. F1644.

I /VE'N m? S (A Z55 yms q)? THUR $6M EN qL 8- 6, 1958 J.,L. AYERS ETAL ,84

ELECTROMAGNETIC WAVEGUIDE SYSTEMS Filed Oct. 27, 1953 v 3 Sheets-Sheet 2 (NVEN ToRS 77 S'IIPHE'A/ MLSH 1958 J. L. AYERS ETAL 8 ELECTROMAGNETIC WAVEGUIDE SYSTEMS Filed Oct. 27, 1953 3 Sheets-Sheet 3 To Matched Termination To Aerial HTTOFZNEY nite States ate ELEcTRoMAGNErrc WAVEGUIDE sYsrEMs James Lee Ayers, Garston, and Arthur Stephen Walsh, Bushey, England, assignors to The General Electric Company Limited, London, England Application October 27, 1953, Serial No. 388,484

Claims priority, application Great Britain November 4, 1952 11 Claims. Cl. 333-11) This invention relates to electromagnetic waveguide systems, and particularly to electromagnetic waveguide systems including what are known as hybrid junctions.

A well known example of a hybrid waveguide junction is that known as the Magic Tee which is, for example, described at page 350 et seq. of Microwave Duplexers, volume 14 of the MIT Radiation Laboratory Series, edited by L. D. Smullin and C. J. Montgomery. Briefly it consists of two T-junctions at the same point along a length of rectangular waveguide, one in the plane containing the short dimension of the cross-section and the longitudinal axis of the length of Waveguide (this is known as the E-plane) "and the other in the plane containing the longer dimension of the cross-section and the longitudinal axis of the length of waveguide (this is known as the H-plane), The Magic Tee waveguide junction is in fact a symmetrical four arm junction at which waves arriving along any one of the arms are coupled equally between the two adjacent arms but not passed into the opposite arm. Various devices have to be fitted in the junction in practice in order to reduce the mismatch introduced by it, and these devices, as

known at present, limit the power of the electromagnetic waves which may be passed through the junction. For

example in one well known arrangement a post and an iris are used to reduce the mismatch, and sparking is particularly liable to occur from the post.

One application of the Magic Tee waveguide junction is in balanced duplexer ararngements for coupling a transmitter and a receiver to a common transmit-receive aerial so that energy originating at the transmitter is not passed into the receiver, or at any rate is passed in such small amounts as to do no damage, and all energy received at the aerial is passed to the receiver. This is described in the reference given above at page 355 et seq. Two Magic Tee junctions are employed, the transmitter being coupled to the E-plane T-arm of the first junction and the aerial being coupled to the H-plane T-arm. The two collinear arms of the first junction are coupled to the corresponding arms of the second junction through equal lengths of Waveguide, the receiver being coupled to the H-plane T-arm of the second junction and the E-plane T-arm of the second junction having a matched termination. T-R cells are inserted in the lengths of waveguide joining the two junctions, at distances from the first junction differing by an odd multiple of A /4 (where a is the wavelength in the waveguide of the waves propagated in it in operation).

In operation waves originating at the transmitter are coupled equally into the two collinear arms of the first junction from the E-plane T-arm. The T-R cells are arranged to fire on the incidence of Waves of a power of the order of those originating from the transmitter, and accordingly present short circuits across the wavelengths which reflect these waves back to the first junction, where they couple in phase into the H-plane T-arm and pass to the aerial. The difierence of the distances to the T-R cells from the first junction mentioned above is provided in order that the phase relationship of the two waves arriving back at the junction shall be correct. On receipt of waves at the aerial, the waves passed down the H-plane T-arm of the first junction divide equally into two collinear arms. These waves are of insutficient power to fire the T-R cells and are therefore passed to the second junction, where, since the arms coupling the two junctions are of equal lengths, they couple in phase into the H-plane T-arm, to which the receiver is coupled, and out of phase into the E-plane T-arm, which has a matched termination. The received waves are therefore passed into the receiver. The arrangement has the additional advantage that if any transmitter energy breaks through the T-R cells it is coupled out of phase into the H-plane T-arm of the second junction and in phase into the E-plane T-arm, the enrgy thus being dissipated in the matched termination of the latter and not passing into the receiver. The power handling capacity of a Magic Tee Waveguide junction using the post and iris matching devices is limited by sparking, particularly from the match ing devices, to about 60 kilowatts at a wavelength in the 3 cm. band using standard waveguide WGl6, that is waveguide having an internal cross-section of 0.9" x 0.4". It is also limited to a similar degree at other wavelengths using appropriately dimensioned waveguides, and using waveguides of other dimensions at the same wavelength. This necessitates a severe limitation of the output power of the transmitter, which is not acceptable in some appli cations for example where the transmitter and receiver are those of a high power pulse radar system.

Another known type of hybrid waveguide junction which may be employed for the same purpose is the ring circuit or rat-race junction. In this type of junction again the maximum power of the waves which may be passed through is of the order of 200 kilowatts in the 3 cms. wavelength band using standard waveguide WGlS, that is Waveguide of internal cross-section of 1.122 x 0.497", the limitation being mainly due to sparking from the corners of the waveguide junctions inherent in the design of the junction.

It is an object of the present invention to provide a hybrid waveguide junction which may be used in balanced duplexer arrangements, and is capable when properly matched of passing waves of greater power than those which may be passed through the known hybrid wave guide junctions.

According to the present invention an electromagnetic waveguide system, including or consisting of a hybrid waveguide junction, includes a first arm in the form of a length of rectangular cross-section waveguide, which divides symmetrically at the junction into a second and a third arm of rectangular cross-section waveguide about a plane containing the direction of its longitudinal axis and the longer dimension of its cross-section at the junction, the longitudinal axes of the second and third arms at the junction being coplanar with that of the first arm and being inclined to one another at an angle not greater than and the longer dimensions of their cross-sections lying parallel to that of the first arm, and a fourth arm opening into the junction at an aperture in one of the shorter dimension walls at the junction of the other three arms, the longitudinal axis of the fourth arm at the junction being perpendicular to the'plane of the longitudinal axes of the other three arms and intersecting that of the first arm, the aperture and at least the end section of the fourth arm next to the aperture being of modified rectangular cross-section (as hereinafter defined), the complete longer dimension side of the cross-section of the aperture being perpendicular to and bisected by the longitudinal axis of the first arm and being close or tangential to the apex formed at the junction of the inner longer dimension walls of the second and third arms, the aperture extending towards the first arm from the complete side and the additional sides of the aperture conforming approximately to the divergent outer longer dimension walls of the second and third arms at the junction.

The term modified rectangular cross-section used in this specification in respect of the cross-section of a waveguide or an aperture is to be interpreted as meaning a cross-section having a shape formed by cutting off symmetrically each of two corners lying at opposite ends of one of the longer sides of a rectangle by an additional side or sides, which may be curved or straight, the other longer side of the rectangle remaining complete. The extent and position of the additional sides is such that :1 TE mode wave, at least approximating to the simplest TE mode wave which may be propagated in a waveguide of triangular cross-section, may bepropagated in a wave guide of modified rectangular cross-section. The term is intended to include within its scope the case in which a triangle is formed by two straight additional sides and the complete longer side of the rectangle.

The system may include matching devices, to match the system for waves arriving at the junction along the first arm or the fourth arm for optimum operation at a desired frequency or over a desired band of frequencies. The matching devices for waves arriving along the fourth arm may include a metal septum projecting from the apex, at the junction of the inner longer dimension walls of the second and third arms, and the shorter dimension wall opposite that in which the aperture lies, and lying in the plane containing the longitudinal axis and the longer dimension of the cross-section of the first arm, and/or a metal post projecting perpendicularly into the aperture from the centre of the complete side. The matching device for waves arriving along the first arm may include an inductive metal iris in the first arm near the junction.

To increase the power handling capacity of the system, the said apex, the edges and the corners of the septum, and the edge or edges of the iris are preferably rounded.

The cross-section of the fourth arm preferably transforms smoothly along its length to a rectangular crosssectiorii The construction and operation of an electromagnetic Waveguide system in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which,

Figure 1 shows a perspective view illustrating the basic form of the system diagrammatically,

Figure 1(a) shows a cross-section of one of the lengths of waveguide in Figure 1, together with a pattern representing the electric vectors in a simple TE mode wave which may be propagated in a waveguide of that crosssection,

Figure 1(b) shows a diagram illustrating the pattern.

of the electric vectors in a simple TE mode which may be propagated in triangular cross-section waveguide,

Figures 2(a)-(c) show sections of a working system in accordance with the present invention,

Figures 3 and 4 show diagrams illustrating the operation of the system, and

Figure 5 shows a perspective view of an arrangement including two waveguide systems each of the form shown in Figure 1.

The basic form of a system inaccordance with the present invention will be described first with reference to Figure 1 of the accompanying drawings, which it will be appreciated is only diagrammati'cal. The system includes a first arm 1 which is a length of rectangular cross-section waveguide and divides symmetrically into a second arm 2 and a third arm 3 about the plane containing its longitudinal axis and the longer dimension of its cross-section. The arms 2 and 3 are-each lengths of rectangular cross-section waveguide the longitudinal axes of which are coplanar with that of the arm 1 and the longer dimensions of the cross-sections of which are power of 500 kilowatts without breaking down.

parallel to that of the arm 1. The actual dimensions of the arms 2 and 3 are equal, but not necessarily equal to those of the arm 1. The longitudinal axes of the arms 2 and 3 are inclined to one another at an angle of 71 in the common plane, each therefore being inclined to the longitudinal axis of the first arm 1 at 35 /2. A fourth arm 4, a length of waveguide having a modified rectangular cross-section terminates at an aperture of the same cross-section in one of the shorter dimension Walls of the arms 1, 2 and 3, the aperture being symmetrically placed over the Y-junction of the arms 1, 2 and 3. In addition the system will usually include various matching devices, but as these may vary from one case to another they have not been shown in Figure l.

The cross-section of the arm 4 and the aperture is shown in Figure 1(a). The shape of the cross-section, which is one form of modified rectangular cross-section as hereinbefore defined, is formed by cutting off two adjacent corners of a rectangle, leaving only the side 5 complete, by the straight additional sides 6 and .7. The remaining sides 8-10 are parts of the sides of the original rectangle. It will be seen from Figure 1 that the side 6 conforms to the diverging outer wall of the arm 2, and similarly, although it is not visible in Figure 1, the side 7 conforms to the diverging outer wall of the arm 3. The dotted line in Figure 1(a) indicates the direction of the longitudinal axis of the arm 1 and this is perpendicular to and bisects the sides 5 and 9. The length of the complete side 5 is equal to the longer dimension of the cross-section of the arm 1, whilst the perpendicular distance between the sides 5 and 9 is equal to the shorter dimension of the same cross-section.

Referring to Figure 1(b) of the accompanying drawings, which shows the pattern of the electric vectors at a point along the waveguide in the simplest TE mode wave which may be propagated along a; waveguide of triangular cross-section, it will be seen that the electric field is mainly concentrated towards the centres of the three sides of the triangle. It is possible to propagate in a waveguide of modified rectangular cross-section a mode similar to the simplest triangular waveguide mode. The electric vector pattern of this mode is shown in Figure 1(a). There is a differencehowever in that for a base of the same size, the cut-off frequency for the triangular waveguide is higher than that for a modified rectangular guide. This is important in this case in that it enables the junction to be usedata lower frequency for given dimensions of waveguide. The details of one hybrid waveguide junction in accordance with the present invention are shown in Figures 2(a)-(c). The junction is designed to operateover a broad band of frequencies,- of 900 mc./s. in the 3 cms. band of wavelengths, in the duplexer of a high power pulse radar system. It has been tested upto a peak transmitter The arm 1- is of standard waveguide WG15, having internal cross-sectional dimensions 1.122 x 0.497, the arms 2 and 3 a short distance from the junction are of standard waveguide WG16 having internal cross-sectional dimensions 0.9 x 0.4", and the arm 4 transforms to standard waveguide WGlS a short distance from the junction.

Figure 2(a) shows a section through the junction in the plane containing the longitudinal axis of the arm 1 and the longer dimension of its cross-section, whilst Figures 2(b) and (0) show sections'at b'b and c-c in Figure 2(a). The junction is shown in Figures 2(a)-(c) as if the various lengths of waveguide ass through a solid block of metal. In fact the block is made up of several constituent parts held together in known manner. Known means may be provided for clamping other lengths of waveguide to the ends of the four waveguide arms 1-4. Referring now to Figures 2(a)(c), the arm l is simply a length of rectangular cross-section waveguide WGlS having internal dimensions 1.122" x 0.497, the longer dimension of the cross-section being parallel to the plane of Figure 2(a). As described above, the arm 1 divides symmetrically about the plane of Figure 2(a) into the arms 2 and 3 the longitudinal axes 11 and 12 of which are inclined at 35 /2 to the longitudinal axis 13 of the arm 1. The cross-sections of the arms 2 and 3 are rectangular. At the junction the internal dimensions are 1.122 x 0.4", but from the points 14 just beyond the junction, up to their ends as shown in Figures 2(a)-(c) the longer dimension tapers to 0.9, the cross-section at the said ends then being those of standard waveguide W616, i. e. 0.9" x 0.4". The inner longer dimension walls of the arms 2 and 3 meet at a rounded apex 16 of 0.217" radius at the junction. The centre of the rounded apex 16 is 0.718 from the point at which the longitudinal axes 11-13 meet. A metal septum 17 projects from the apex 16 towards and into the arm 1, the length of the septum 17 being 0.867 from the tip of the apex 16 to the tip of the septum 17. The height of the septum 17, 0.360, is rather less than half the longer dimension of the cross-section of the arm 1. The septum 17 is rounded at the corner 18, the radius being and is 0.125" thick. The narrow edge of the septum 17 is also rounded, the radius being 0.0625".

The arm 4 opens into the other three arms 1-3 at the aperture 19 in the left hand (as situated in Figure 2(a)) narrower dimension wall of the arm 1, 2 and 3 at the junction. Its longitudinal axis 21) is perpendicular to the plane containing the longitudinal axes 1113 of the arms 1-3. Its cross-section at the aperture 1% is modified rectangular and has previously been described with reference to Figure 1(a). The walls corresponding to the sides 5, 6, 8 and 9 of Figure 1(a) maybe seen in Figure 2(a) and are given the same references. From the point 21 along its length the cross-section transforms smoothly to a rectangular cross-section at the point 22 along its length, of the same internal cross-section dimensions as the arm 1. The transformation is effected by a sloping ramp 23 starting at the end of the side 6 and tapering to an apex Z4, and a similar symmetrically placed ramp not visible in Figure 2(a) in the adjacent corner of the arm 4 tapering away from the end of the side 7 at the same point 22 along the length of the arm 4.

An inductive iris 25" is provided in the arm 1, spaced 0.616" from the apex 16. The iris 25 is 0.171 high and 0.05" thick and extends across the left hand (as seen in Figure 2(a)) shorter dimension wall of the arm 1. The edge, which projects into the arm 1 is rounded.

At the point where the outer longer dimension walls start to diverge, the right hand half (as seen in Figure 2(a)) of those walls are curved circularly. Owing to the fact that the additional sides 6 and 7 of the modified rectangular cross-section of the aperture 19 and the first part of the arm 4 only approximately conform to these diverging Walls, facets 26 are formed in the left hand halves. These facets 26 are curved at the points half way across the walls with the same radius as the circularly curved parts, but converge to points at the corners 27 of the aperture 19.

The positions and magnitudes of the septum 17 and the iris 25 are determined so that waves arriving at the junction along the arm 4 or the arm 1 respectively are divided equally into the arms 2 and 3 with the minimum mismatch.

It will be appreciated that the junction just described acts as a symmetrical four arm junction by virtue firstly of the fact that it has a plane of symmetry, i. e. that containing the longitudinal axes of the first and fourth arms 1 and i. This symmetry results in cross-attenuation between the arms 1 and 4.

Further as stated above, the septum 17 and the iris 25 are introduced to reduce the mismatch at the junction for waves arriving along the arms 4 and 1 respectively.

When these arms 1 and 4 are effectively matched by this means, the system is automatically matched in arms 2 and 3 in addition, and also cross-attenuation is introduced between the arms 2 and 3. These properties arise solely from the configuration of the junction and the introduction of the matching devices. In addition, due to the configuration, there is an accurate, constant and non-frequency sensitive split of power equally between the arms 2 and 3 when waves reach the junction along either of arms 1 and 4. The septum 17 does not interfere with the passage of waves from the arm 1 into the arms 2 and 3 since it lies in a plane perpendicular to the electric field of these waves.

The waveguide system described above may be used in a duplexer system as shown in Figure 5. T-R cells 3d and 35 are inserted in the arms 2 and 3 or extensions thereof at electrical distances from the junction at (which is of the form previously described) differing by one quarter of the wavelength in the waveguide of the waves passed through the system in operation. The receiver (not shown) of the duplex system is coupled to the two arms 2 and 3 beyond the TR cells 34 and 35 in a manner to be described. The transmitter (also not shown) is coupled to the arm 4, the coupling being arranged so that waves in the TE mode are excited in the rectangular part of the arm 4 by the transmitter and are propagated towards the junction 36. Similarly the aerial (not shown) is coupled to the arm 1 so that waves received on the aerial excite waves in the TE mode in the arm 1 which are also propagated towards the junction 36.

Considering first waves received on the aerial, the junction 36 of the arms 1, Z and 3, being in the E-plane of those arms, acts, as mentioned above, as a simple power divider, the waves arriving at the junction 36 along the arm 1 being coupled equally into the two arms 2 and 3. They will usually be of insufiicient power to fire the T41 cells 34 and 35 and are passed through them into the receiver. There is no coupling of these waves into the arm 4 since the pattern of the electric and magnetic vectors across the mouth of the arm 4 is unsuitable.

This is illustrated in Figure 3 of the accompanying drawings, which shows a diagram of the waveguide system marked with arrows representing the direction of the electric vectors at one maximum of a wave which travels down the arm 1 and divides into the arms 2 and 3. There is no tendency to excite waves in the arm 4 as the distribution of the electric vectors across the aperture does not in any way resemble that of the electric vectors shown in Figures 1(a) and ([2).

Referring once again to Figure 5 consider now waves originating at the transmitter, these pass down the rectangular section of the arm 4 as waves of the TE mode. In the transformation section between the points 22 and 21 along the arm 4, the waves are transformed to a mode, in the length of modified rectangular crosssection waveguide, approximating to a triangular waveguide mode as illustrated in Figures 1(a) and (b). Reference will now be made in addition to Figure 4 of the accompanying drawings, which shows a diagram of the waveguide system marked similarly to Figure 3 with arrows representing the direction of the electric vectors at one maximum of a wave travelling down the arm 4 and passing through the junction to the arm 1. The mode of the waves in the arm 4 is represented roughly by the two full line arrows 30. As will be seen from Figure 4 this mode tends to result in the propagation of waves in the T E mode along the arms 2 and 3 and away from the junction as represented by the arrows 31. The excitation of waves in the arm 1 is inhibited since the electric vectors represented by the two arrows 32 of waves which are excited, are in opposite directions and the fields therefore cancel.

The power of these waves originating at the transmitter is sufficient to fire the T-R cells 34 and 35, and the waves propagated along the arms 2 and 3 are reflected back towards the junction 36. These reflected waves are represented by the dotted arrows 33 (Figure 4). As the distances of the T-R cells 34 and 35 from the junction 36 differ by A 4, one of the waves undergoes effectively a phase reversal, which is shown by reversing the direction of the arrows in the arm 2. The waves therefore combine at the junction 36, as illustrated by the dotted arrows in Figure 4, to excite a single wave of the TE mode in the arm 1. The distribution of the electric vectors across the aperture 19 due to these reflected waves is not suitable to excite a wave in the arm 4. In this way therefore it will be seen that the waves originating at the transmitter are coupled into the arm 1.

The duplexer system of Figure 5 is completed by a further waveguide junction 37 of similar construction but constructed throughout of waveguide WG16 having internal cross-section 0.9 x 0.4". The arms 2 and 3 of the junction 37 being coupled to the same arms of the junction 36 by equal lengths of rectangular cross-section waveguide at intermediate points along which the TR cells 34 and 35 are inserted at distances from the junction 36 differing by x /4. The receiver is coupled to the first arm 1 of the junction 37 and the fourth arm 4 has a matched termination. The received waves passed through the TR cells 34 and 35 are then coupled into the receiver, and any brealothrough waves from the transmitter are coupled into the matched termination of the fourth arm 4. it will be appreciated that the resulting configuration is very convenient, particularly where the system has to be fitted into a small space. The arms 2 and 3 of each junction 36 and 37 may for example be bent back through 35 /2 until their longitudinal axes are parallel to those of the arms 1. The two junctions 36 and 37 are therefore joined by two lengths of waveguide running parallel to one another and in which the T-R cells 34 and 35 are mounted. The arm 1 of the junction 37 is coupled to the receiver and the arm 1 of the junction 36 to the aerial. The transmitter is coupled to a fourth arm 4 of the junction 36 to which the aerial is coupled. It will be appreciated further that the junction 36 to which the receiver is coupled need not have a high power handling capacity and therefore differences of design can arise, such for example as the use of waveguide WG16 throughout.

The junction described above with reference to Figure 2(a)-(c) of the accompanying drawings may be subject to variations in design. The arms 1 and 4 are matched by means of the iris 25 and the septum 17, but other forms of matching device may be employed instead of or in addition to the two described although it was only possible to include the post in view of the much smaller power handling capacity required. Thus, for example, in the smaller junction employing waveguide WG16 throughout used as the second hybrid junction in the duplexer described, a post is included projecting a short distance into the aperture 19 from the centre of the side 9. This was included to improve still further the matching of the arm 4. Further for the same purpose the side 9 may be positioned so that a small part of the apex 16 projects into the aperture 19. Another important factor in the matching of the junction is the position of the apex 16 with respect to the junction of the longitudinal axes 11-13 of the arms 1-3.

The junction described with reference to Figure 2(a)- (c) is designed to have a high power handling capacity and in that connection the rounding of the apex 16, the edge and corner of the septum 17 and the edge of the iris 25 are regarded as important. As compared with a Magic Tee junction, the use of the Y configuration of the arms 1, 2 and 3 instead of a T-junction is also important in this respect.

Further, a technique of die-casting a quasi-rectangular waveguide has recently been developed, for simplifying the manufacture of waveguides and waveguide systems. The waveguide resulting has a cross-section in which the shorter dimension of the cross-section decreases from the centre to the edges, the longer edges sloping at a small angle, say 5, whilst the corners are rounded at a radius of for example 0.05. The whole of a length of this waveguide is cast in two equal parts, divided along the line bisecting the longer sides, the parts being joined together in assembly. The shape departs so slightly from rectangular that waves may be propagated in modes almost identical with those of rectangular cross-section waveguide. It is to be understood that where rectangular cross-section waveguide is referred to in this specification, the term is intended to include waveguide of quasi-rectangular cross-section.

The configuration at the junction may vary in other respects. The aperture 19 may have curved additional sides, for example it may have curved sides forming exactly to the divergent outer longer dimension walls of the arms 2 and 3 instead of the sides 6 and 7 which, being tangential, only conform approximately. In this case the arm 4 may have first a length in which the crosssection transforms smoothly from that of the aperture 19, to one with straight additional sides, and then a further length in which the cross-section transforms smoothly to rectangular. Ideally the extreme case in which the rectangular cross-section is modified to become triangular provides the best matching, but the cut-off frequency for triangular waveguide of the appropriate size is too high and may undesirably limit the lowest frequency of operation.

Provided a suitable means is provided for exciting waves in the correct mode in the waveguide of modified rectangular cross-section it would not of course be necessary to transform the cross-section of the arm 4 at all, and a uniform length of the modified rectangular cross-section could be employed.

On testing a junction such as that described with reference to Figures 2(a)(c) of the accompanying drawings, the following figures were obtained,

The voltage standing wave ratio (V. S. W. R.) for waves passed into the arm 1, with matched terminations on the end of arms 2 and 3 was greater than 0.90 over a frequency band of 1050 mc./s. in the 3 cms. band of wavelengths.

The V. S. W. R. for waves passed into the arm 4, with T-R cells correctly positioned in arms 2 and 3 and a matched termination on arm 1 was greater than 0.90 over a frequency band of 500 mc./s. in the 3 cms. band of wavelengths. A peak power of 500 K-watts was passed into the arm 4 without breakdown.

The cross-attenuation between arms 1 and 4 was greater than 45 decibels over the frequency band of 1000 mc./s.

In the junction employing Waveguide WG16 throughout and including a post matching device, the bandwidth for V. S. W. R. greater than 0.9 is increased to 1800 mc./s. for arm 1 and 1350 mc./s. for arm 4.

We claim:

1. An electromagnetic waveguide system, including a hybrid waveguide junction, the system including a first elongated arm in the form of a length of waveguide of oblong cross-section, said first arm thereby including two parallel broad walls and two parallel narrow walls, said first arm dividing symmetrically at the junction into a second elongated waveguide arm and a third elongated waveguide arm, said second and third waveguide arms being of oblong cross-section and thereby each including two parallel broad walls and two parallel narrow walls, said narrow Walls of the second and third arms merging into the narrow walls of the first arm at the junction, said first arm dividing at the junction about a plane between and parallel to the two broad walls of said first arm, the longitudinal axes of the second and third arms at the junction being coplanar with that of the first arm and being inclined to one another at an angle not greater than so that the outer broad walls of the second and third arms are divergent at the junction, the longer cross-section dimensions of the second and third arms lying parallel to that of the first arm, an aperture in the narrow walls of the first three arms at the junction of said arms and a fourth elongated waveguide arm opening into the junction at said aperture, the longitudinal axis of the fourth arm at the junction being perpendicular to the plane of the longitudinal axes of the other three arms and intersecting that of the first arm, the aperture and at least the end section of the fourth arm next to the aperture having a six-sided shape that is symmetrical on either side of the plane containing the longitudinal axes of both the first and fourth arms, saidsix-sided shape including a first pair of parallel walls one of which is longer than the other and both of said walls crossing the plane of symmetry of the aperture, a second pair of parallel walls perpendicular to the walls of the first pair and extending from the longer wall thereof, and the remaining two walls conforming approximately to the two divergent walls of the outer broad walls of the second and third arms at the junction respectively and extending from the shorter of the first pair of walls to the second pair of walls.

2. An electromagnetic waveguide system according to claim 1, including a matching device to match the system for waves arriving along the fourth arm, the device being in the form of a metal septum projecting from the apex, at the junction of the inner broad walls of the second and third arms, and the narrow wall at the junction opposite to that in which the aperture lies, the septum lying in the plane containing both the longitudinal axis and the longer cross-sectional dimension of the first arm and the height and shape of the septum being determined for optimum operation of the system over a desired band of frequencies.

3. An electromagnetic waveguide system according to claim 2, in which all edges and corners of the septum projecting into the waveguide system are rounded to increase the power handling capacity of the system.

4. An electromagnetic Waveguide system according to claim 1, including a matching device to match the system for waves arriving along the first arm, the device being in the form of an inductive metal iris in the first arm near the junction, the position and dimensions of which are a determined for optimum operation over a desired band of frequencies.

5. An electromagnetic waveguide system according to claim 1, in which the apex at the junction of the inner broad walls of the second and third arms is rounded.

6. An electromagnetic waveguide system according to claim 5, in which the complete side of the aperture is tangential to the rounded apex.

7. An electromagnetic waveguide system according to claim 1, in which the divergent outer broad walls of the second and third arms curve smoothly at the junction to meet the broad walls of the first arm, and the modified rectangular cross-section of the aperture is such that it has two additional straight sides which are tangential to the curved walls of the second and third arms.

8. An electromagnetic waveguide system according to claim 7, in which the cross-section of the fourth arm first transforms smoothly betwen two points along its length to a modified rectangular cross-section having two straight additional sides and then transforms smoothly between two further points along its length to a rectangular crosssection.

9. An electromagnetic waveguide system according to claim 1, in which the divergent outer broad walls of the second and' third arms curve smoothly at the junction to meet the broad walls of the first arm, and the modified rectangular cross-section of the aperture is such that it has two additional curved sides conforming exactly to the curved walls of the second and third arms.

10. An electromagnetic waveguide system according to claim 9, in which the cross-section of the fourth arm transforms smoothly between two points along its length to a rectangular cross-section.

11. A balanced duplexer, for coupling a transmitter and a receiver to a common waveguide feeder, comprising a first system in accordance with claim 1, designed to pass powers greater than the peak power of the trans mitter, and a second system according to claim 1, the transmitter being coupled to the fourth arm of the first system, the common waveguide feeder being coupled to the first arm of the first system, the receiver being coupled to the first arm of the second system, a matched termination being coupled to the end of the fourth arm in the second system, and the second and third arms of the two systems being coupled by equal lengths of waveguide, in each of which is situated a T-R cell at distances from the junction of the first system which differ by an odd number of quarter wavelengths at the frequency at which the system is designed to operate.

References Cited in the file of this patent UNITED STATES PATENTS Pratt Sept. 25, 1956 

